The present invention relates to a system for controlling the injection of fuel for a two-cycle engine with a direct fuel injection system.
There are various methods for supplying fuel to the two-cycle engine. In a method where fuel is mixed with air to make a mixture and the mixture is supplied to a cylinder of the engine, a part of the fuel escapes from the cylinder through an exhaust port upon scavenging of the cylinder. In order to prevent the fuel from escaping through the exhaust port, there is a method in which air alone is delivered to the cylinder and the fuel is injected directly to the cylinder by an injector. In such an engine with the direct fuel injection system, when scavenging is performed in the cylinder, a part of scavenge air is discharged through the exhaust port together with the burned mixture. Consequently, the air-fuel ratio of the mixture in the combustion chamber is different from the air-fuel ratio estimated from a constituent of the exhaust gas. Accordingly, it is difficult to control the air-fuel ratio of the mixture based on the constituent, of the exhaust gas detected by an O.sub.2 -sensor provided in the exhaust system.
It is also necessary to control the air-fuel ratio for improving and stabilizing exhaust emission of the two-stroke-cycle engine. However, an open-loop control cannot control the air-fuel ratio with accuracy, and hence, it is preferable to control the air-fuel ratio with feedback control.
Japanese Patent Application Laid-Open 62-113819 discloses a two-stroke-cycle engine in which first and second scavenge passages are provided. The first scavenge passage is provided with a first throttle valve and a fuel injector. The quantity of fuel is calculated in accordance with engine speed and a signal from an air-flow meter. However, the engine is not employed with a direct fuel injection system, but fresh air is delivered from the first and second scavenge passages to perform scavenging so as to prevent the fuel from escaping through the exhaust port. Accordingly, the structure and control system of the engine are complicated. Further, since the quantity of injected fuel is calculated in accordance with engine load, the air-fuel ratio is not controlled.